Overnight tea, poisonous as a snake? Overnight food sent you to the ICU?

Overnight tea, overnight water, overnight food... some people think it is no big deal, but some people believe that as long as the word " overnight " is involved, it will be harmful to the body.

Can ordinary tea and food really cause great toxicity after being left overnight? Today we will use three experiments to find out.

Overnight tea

Some people say that " overnight tea is as poisonous as a snake ", and some people worry that overnight tea may produce nitrite. So, can we drink overnight tea? What is the truth?

In 2014, the CCTV Finance program "Is It True?" investigated this claim and invited senior tea tasters to taste it. The conclusion was that overnight tea is safe and drinkable .

The experiment selected three cups of tea: Longjing, Pu'er and Chrysanthemum, and left them overnight.

turn out:

• Overnight tea has not deteriorated and has no rancid taste . Whether the overnight tea is left for 12 hours or 24 hours, it still has the taste that the tea should have.

• Nitrite does not exceed the standard . After testing, the nitrite content in overnight tea is far lower than the sanitary standard for drinking water: ≤1mg/L.

Because tea is rich in tea polyphenols and vitamin C, they hinder the formation of nitrite.

Therefore, overnight tea or tea that has been brewed for too long can be drunk as long as it has not deteriorated.

Public nutritionist Chang Lijie reminds: If there is no microbial contamination, overnight tea can be drunk from a safety perspective, but it is not recommended from the perspective of nutritious tea . This is because the tea polyphenols in the tea undergo oxidation reactions, which darkens the color of the tea soup, affects the senses, and reduces nutrients such as polyphenols and vitamin C.

Overnight water

We often hear that " you should never drink boiled water or overnight water because the nitrite content increases and drinking such water frequently can lead to poisoning or even cancer ." But is this really the case?

In 2020, staff from the Food Inspection Center of the Market Supervision Bureau of Weihai City, Shandong Province conducted a special experiment to restore the truth using data.

They used a stainless steel electric kettle to heat tap water, boiled it 1, 5, and 10 times, then poured it into a cup and allowed it to cool naturally. They then tested the nitrite content in the water after leaving it for 0, 24, and 48 hours.

The conclusion is: the nitrite content does not exceed the standard, but it is recommended that the water should not be left for drinking for more than 24 hours .

The experiment found that as the number of heating and boiling times increased, the nitrite content in the water tended to increase slightly.

As the storage time increases, the nitrite content in the water increases significantly. After 24 hours, the nitrite content increases by 2-4 times. After 48 hours, the maximum nitrite content is 0.014 mg/L .

However, the reference index and limit value of nitrite in drinking water in my country's national standard is 1mg/L. So from this point of view, it is safe to drink.

The increase in nitrite is because tap water contains nitrate. After repeated boiling, a small amount of nitrate will be converted into nitrite due to high temperature and lack of oxygen. At the same time, the evaporation of water during the heating process will also cause the nitrite concentration to increase.

Wang Dongxu, a Chinese medicine doctor and director of the Department of Geriatrics at Nanjing Hospital of Integrated Traditional Chinese and Western Medicine, said that nitrites do exist in water that has been stored or boiled for a long time, but the content is very low . Some data show that there are only more than 100 micrograms per liter. If the amount is harmful, you would have to drink at least one ton. Moreover, nitrites themselves are not carcinogenic. They are carcinogenic only when they are converted into nitrosamines. This conversion requires many prerequisites, and it is not possible to generate them by boiling water.

However, although "overnight water" can be drunk, considering the problem of bacterial contamination, it is recommended that it should not be left for more than 24 hours. And after the water is boiled, try to cover it with a lid to prevent bacterial contamination .

When boiling water, try not to take too much at one time. Take only what you need. If there is any leftover, you can heat it again and continue drinking it.

Overnight meals

Many people often encounter these situations. They cook too much and don't want to waste it, so they put it in the refrigerator and continue eating it the next day. Some people also save trouble and cook enough for several days at a time... However, there are also news reports of people eating leftover food and being admitted to the ICU.

First of all, it should be made clear that overnight food does not only refer to food that has been left overnight. If it has been left for more than 8-10 hours, it should be considered "overnight" .

There are two main reasons why toxic ingredients in overnight dishes increase:

One reason is that the chemical substances in the food produce carcinogens, such as nitrite, which cannot be removed even by heating; the other reason is that it is contaminated by foreign bacteria when left alone .

So can we eat leftover food? In 2019, Ningbo Food Inspection and Testing Institute conducted a scientific experiment on leftover food, using 30 dishes and 450 test data to solve the mystery of leftover food.

They collected freshly cooked dishes (meat dishes, semi-vegetarian dishes, vegetarian dishes, and salad dishes) and placed them in a 4°C environment (simulating a home refrigerator) and a 25°C environment (simulating room temperature). Finally, they tested the nitrite and bacterial content.

The following conclusions were drawn from this experiment:

• Put the food you cooked at noon in the refrigerator and eat it in the evening

From the experimental results, it can be seen that when dishes are refrigerated and stored at 4°C, the nitrite content remains basically unchanged after 6 hours, and the microbial growth is relatively small.

• Keep the food cooked at noon at room temperature and don’t eat it at night

The results of the experiment showed that after six hours of storage at 25°C, the nitrite content of the dishes stored at room temperature did not change much, but the microorganisms began to proliferate, and the number of colonies increased significantly compared to when they were just out of the pot. And cold dishes are more likely to cause microbial proliferation .

• If you want to eat the food you cooked at noon or in the evening again, you must put it in the refrigerator

Judging from the experimental results of Ningbo Food Inspection Institute, the nitrite content of overnight dishes refrigerated and stored at 4°C, whether it is meat or vegetables, remains basically unchanged within 24 hours, and the increase in the total number of colonies is also very small. Therefore, there is no risk of excessive nitrite, excessive total number of colonies, or toxicity in overnight dishes .

The fact that leftover food is eaten in the hospital is often due to improper storage methods and time. Zhang Xiaoyan, director of the Nutrition Department of Lianyungang Maternal and Child Health Hospital in Jiangsu Province, said that the process of leftover food being contaminated by bacteria has nothing to do with whether it is left overnight, but only with whether the food itself has deteriorated, and of course it is also related to the production process and storage conditions .

Finally, I would like to remind everyone to pay attention to 3 points for overnight dishes:

• Most overnight dishes can be eaten, but try to eat less, cook according to the quantity, and try not to have leftovers . Whether it can be eaten or not mainly depends on the storage conditions and storage time.

• If you cannot avoid eating overnight food in your daily life, you must also use low-temperature storage . When placing leftovers in the refrigerator, be sure to store them separately to avoid cross contamination. You can use clean covered containers or cover them with plastic wrap.

• When eating overnight food, make sure to reheat it thoroughly .